6.3.2 Compressor
Radial compressors are generally used to handle the air flow rate and pressure ratio of gas turbines in the SMGT class, and they also have a simple structure. However, in order to achieve the high isentropic efficiency of 84%, the SMGT uses the axial type for the low pressure stage and the radial type for the high pressure stage.
The axial compressor passage requires a smaller radius from the intake side to the outlet in order to connect it to the radial compressor. Because this causes a drop in the circumferential speed in the after-stage and a drop in the aerodynamic loading, a four-stage axial compressor is used to achieve a high isentropic efficiency. Futher, at the first stage the maximum relative inflow Mach number is kept below the speed of sound to reduce the blade loss associated with increasing mach speeds.
Between the radial and axial stages a short, straight passageway is used to minimize loss. To prevent loss caused by an increase in mach speed of the relative inflow to the radial impeller, swirl is left at the outlet of the axial stage, causing pre-swirl at the radial stage inlet. This keeps the relative inflow Mach number at the impeller's inlet shroud side to about Mach 0.7, a comparatively low figure.
